Selective laser sintering (“LS”) is a layer-wise additive manufacturing technique in which electromagnetic radiation, for example from a CO2 laser, is used to bind a powder building material at select points to create a solid structure having a desired three-dimensional shape. The building material may include plastic, metal (direct metal laser sintering), ceramic, or glass powders. In some related techniques, for example techniques for use with metals, a technique referred to as selective laser melting (“LM”) is used in which the powder is melted as opposed to sintered. While there are similarities between the processes, there are also distinct differences, both in the processes and powders used therein.
Typically, a CAD model of an object to be constructed by LS is prepared using known software applications. The CAD model is sectioned into layers to create build data for the object. The build data comprises a plurality of cross-section patterns according to the CAD model. For each layer the LS building process is as follows: a layer of a building material is deposed on a bed of a laser sintering system. The applied layer is scanned and solidified at select points by a source of controlled electromagnetic radiation in accordance with the cross-section pattern associated with that layer. After a cross-section is scanned causing solidification at select points, the bed is lowered by one layer thickness, a new layer of powdered material is deposed on the bed, and the bed is rescanned by the laser. This process is repeated until the build is completed. Prior to scanning, an LS machine may preheat the powder material deposed on the bed to a temperature proximate to a melting point of the powder. Preheating is typically accomplished by one or more radiant heaters. Preheating the powder makes it easier for the laser to raise the temperature of powder to a fusing point.
After the layer-wise process is completed, the formed object(s) is deposed in a volume of unfused powder, referred to as a cake. The formed object(s) is extracted from the cake and unfused powder is removed from any voids in the object. Compressed air may be used to aid in this step. The powder from the cake that is not fused into the built part can be recovered, sieved, and used in a subsequent LS build.
A known problem that arises during the layered production of objects from powdered materials is that the physical properties of the object being formed vary from step to step with each melted volume element. This can decrease the mechanical strength of the object in the z-axis. One reason for this, particularly in cases of LM of metals, is the change in the thermal conductivity caused by the increase in the solidified volume relative to the unsolidified volume during the layer-wise building process. In the case of materials having a relatively high thermal conductivity, the increase in temperature brought about by a specific application of energy per time unit to the respective irradiation point depends on the thermal conductivity of the surrounding region of the irradiation point.
In the case of metals, there is a problem that the thermal conductivity of the powdered material often differs considerably from the thermal conductivity of a formed region of the constructed objects, which material has been solidified by melting during the building process. If the respective irradiation point is surrounded exclusively by powdered material the heat formed at the irradiation point cannot dissipate very effectively and localized overheating of the powdered material may occur, resulting in reduced mechanical strength of the constructed part. On the other hand, if the irradiation point is surrounded by solidified material the heat can dissipate more rapidly because of the higher thermal conductivity of the surrounding region and the irradiation point is not so easily overheated. Based on these effects it was often the case that different regions of a constructed object were melted at considerably varying temperatures, resulting in the formation of mechanical stresses in the object.
U.S. Pat. No. 5,427,733 to Benda et al. (“Benda”) discloses a method for performing temperature controlled laser sintering of metal. Benda attempts to solve the above problem associated with LM of metal by dynamically adjusting the power of the laser. Benda discloses a temperature-controlled laser sintering system that controls the power of the laser as a function of the temperature of the bed surface. The system includes a laser beam which is focused onto a sintering bed by a focusing mirror and a set of scanning mirrors. Thermal radiation emitted from the sintering bed is imaged to the scanning mirrors and to a dichroic beam splitter which reflects such radiation but passes the wavelength of the laser beam. The radiation is focused onto an optical detector which provides a signal on a line to a power control circuit. The power control circuit controls a modulator which modulates the power of the laser beam so as to maintain the thermal radiation emission (and thus the temperature at the sintering location) at a substantially constant level.
U.S. Publication No. 20140332507 to Fockele (“Fockele”) is also directed to the manufacture of objects from metal using layer-wise LM construction methods. Fockele recognizes the necessity of considering temperature inhomogeneities within a layer to be solidified when energy is inputted by means of a laser. Fockele teaches accounting for different heat dissipation capabilities of the surrounding area of a point of incidence of the beam and calculating the heat dissipation capability for each voxel in this local surrounding area. In order to determine the heat dissipation capability at a point of incidence of the beam, Fockele teaches accounting for heat dissipation capability in an area preferably over at least 100 layers in a downwards direction below the current irradiation point and preferably at least as wide.
U.S. Publication No. 20160332379 to Paternoster et al. (“Paternoster”) is also directed to a method of controlling energy in a layer-wise construction method. Paternoster attempts to minimize a variation in remelting temperature by considering the time dependence of the cooling-down of a solidified region. The adjustment method taught in Paternoster is premised on the observation that for longer total exposure times of the regions exposed (solidified) within a layer, these regions have more time for giving off heat by heat radiation, heat conduction or convection. Therefore, the layer cools down more and there is not so much energy available in total for a melting or sintering of the following layer. Paternoster postures that as a result, the mechanical properties such as the elongation at break, are worse due to a worse adhesion of the following layer. On the other hand, for a short total exposure time of a layer, such layer does not cool down so much and the powder in the following layer can melt more completely resulting in better mechanical properties. In order to overcome this problem, Paternoster discloses adjusting the heat inputted per unit area for a solidification in dependence of the total exposure time of the region to be solidified in the layer lying below the layer to be currently exposed or in dependence of the total exposure time of the region to be solidified in the layer currently to be exposed.
A disadvantage of the solutions proposed is that they adjust the heat inputted per unit area for a solidification in dependence of the thermal conductivity of the building material. Further, the solutions proposed above are concerned with LM of metal powder, which has a relatively high thermal conductivity in a solidified state and a relatively lower thermal conductivity in a powder form. For example, a recent study titled, Thermal Conductivity of Metal Powder and Consolidated Material Fabricated Via Selective Laser Melting, found that a stainless steel in powder form has a thermal conductivity of 0.14 W/(m*K) and the material in bulk form has a thermal conductivity of 42.70 W/(m*K).
The discussed methods are not applicable for LS of a polymer material that exhibits significantly less variation in thermal conductivity between powdered form and bulk form. For example, polyaryletherketones (“PAEK”) polymers are of specific interest in the LS process because parts that have been manufactured from PAEK powder or PAEK granulates are characterized by a low flammability, a good biocompatibility, and a high resistance against hydrolysis and radiation. The thermal resistance at elevated temperatures as well as the chemical resistance distinguishes PAEK powders from ordinary plastic powders. A PAEK polymer powder may be a powder from the group consisting of polyetheretherketone (“PEEK”), polyetherketone ketone (“PEKK”), polyetherketone (“PEK”), polyetheretherketoneketone (“PEEKK”) or polyetherketoneetherketoneketone (“PEKEKK”). For example, PEKK is of interest in the LS process. Available publications and test data show that the thermal conductivity of powder PEKK is 0.1 W/(m*K), while the thermal conductivity of bulk PEKK is approximately 0.2 W/(m*K).
Another disadvantage of the known methods of adjusting the laser power in layer-wise building techniques is that they are agnostic to part geometry. In the LS of a polymer, for example PEKK, the powder is deposed on the bed surface having a thickness of approximately 125 μm. Use of the normally rated laser power for a PEKK material at this thickness during LS causes penetration into sub layers of the LS bed, thereby providing additional heat energy to the layers directly below the point of incidence. The area below the point of incidence that is affected by the energy input by the laser is commonly referred to as the heat affected zone or HAZ. In the case of PEKK and other PAEK polymers, the HAZ is typically between 1 and 5 layers below the point of incidence, depending on the laser power, the material, and the thickness of the layers.
The inventors have discovered that the HAZ can have significant impact on part geometry, particularly as it relates to the construction of voids in the object for construction. For example, when constructing such voids in an object a plurality of layers having a continuous area of unsintered material are deposed on bed. The unsintered material will ultimately be removed when the build is complete, leaving a void that is defined by the surface of the surrounding material sintered during the process. Typically, the void will be defined by an upper surface.
A disadvantage of the known methods of LS is that when sintering a downskin layer the HAZ will cause layers below the downskin to become sintered and/or to adhere to the object that is being constructed, causing the geometry of the complete part to materially deviate from the geometry specified by the CAD model. This effect can be particularly disadvantageous in the manufacture of parts requiring tight tolerances. For example, Oxford Performance Materials, Inc., based in S. Windsor, Conn., uses the LS technique to manufacture customer medical implants from PEKK, including its OsteoFab® Patient Specific Cranial Device (OPSCD), OsteoFab® Patient Specific Facial Device (OPSFD), and OsteoFab® Patient Specific Facial Device (OPSFD). Oxford Performance Materials, Inc. also manufactures aerospace components and industrial components using the LS layer-wise method. In these applications, ensuring that the constructed part conforms geometrically to its CAD model is essential. After a part is constructed, its geometry can be compared to the CAD model using a number of different techniques.
Another disadvantage of the known solutions is that they do not account for reverse heating in the area of underhang, i.e., the reverse of an overhang. Reverse heating causes unwanted interlayer fusion in subsequent layers. Although less pronounced, the latent heat in a recently sintered portion of a layer will cause an immediately adjacent and unsintered portion in a subsequent layer applied above to become adhered to the underhang, resulting in an additional deviation in part geometry.
Accordingly, there is need for an improved system and method for the manufacture of an object by LS.